[NFC] [FlowSensitive] Fix missing namespace in MockHeaders (#170954)

This happened to work because we were missing both a namespace close and
open and things happened to be included in the correct order.

Author: fmayer

clang/unittests/Analysis/FlowSensitive/MockHeaders.cpp

@@ -1358,6 +1358,8 @@ bool operator!=(const Status &lhs, const Status &rhs);
 Status OkStatus();
 Status InvalidArgumentError(const char *);
 
+}  // namespace absl
+
 #endif // STATUS_H
 )cc";
 
@@ -1370,6 +1372,8 @@ constexpr const char StatusOrDefsHeader[] = R"cc(
 #include "std_type_traits.h"
 #include "std_utility.h"
 
+namespace absl {
+
 template <typename T> struct StatusOr;
 
 namespace internal_statusor {